Or to put it another way...
Do you spend $800 on a 24/2.8 IS and another $850 on a 28/2.8 IS or $1300 on a Tamron 24-70/2.8 VC?
Indeed, with that one lens, Tamron has taken the wind out of the sails of four different Canon lenses, to one extent or another -- and two of the four are or have the potential to be legendary.
The original 24-70 L? More expensive than and not as good as the Tamron, plus the Tamron's got IS.
The new 24-70? Presumably better than the Tamron, but still no IS, costs twice as much, and still not yet available.
And each of these primes? Looks like they're in the same ballpark as the Tamron...but they're not that much cheaper, and the Tamron gets you the same focal lengths as both, plus
all the way out to 70mm.
For that matter, the Tamron's even (at least on paper) competing well with the 24-105. Sure, you lose out on the long end, but you get the extra stop and
you keep the IS.
Oh -- and add me to the list of those who don't see the point to a slow wide prime with IS. At ISO 100, 1/30s and f/2.8, you're already down to EV 8, which is "Las Vegas or Times Square at night. Store windows. Campfires, bonfires, burning buildings. Ice shows, football, baseball etc. at night. Interiors with bright florescent lights." ISO 3200 gets you to fireworks. 1/30s is already the rule of thumb for handholdability at these focal lengths, and it's too slow for people (mostly)...so the IS is only really useful for handheld moonlit landscape photography or (so I hear) for video. Compare that with f/1.4 which would get you up to 1/125s in the same scene where you'd be at 1/30s at f/2.8, and you can really see the advantage of fast glass over IS for low light.
I'm also not seeing enough of a difference between 24mm and 28mm to understand why Canon thought they needed both.
Ah, well. I wish Canon the best with these new lenses, but I'm much more excited by my brand new Shorty McForty than I'd ever be by either of these new primes.